1. Field of the Invention
This invention relates to releasing agents and more particularly, to a solvent-free releasing silicone composition which is suitable for application to a substrate such as paper with or without lamination, plastic films or the like, in order to impart releasing or non-adhesive or tack-free properties to the substrate.
2. Description of the Prior Art
Various types of solvent-free releasing silicone compositions have been proposed to prevent adhesion between one side of a substrate such as paper or a plastic film or sheet and an adhesive layer applied on another side of the substrate. For instance, Japanese Laid-open Patent Application Nos. 49-47426, 50-141595, 52-34791 and 57-77395 describe these types of compositions.
However, these known releasing silicone compositons have the drawback that although they exhibit good adhesion to papers, laminated papers and metal foils, it takes a relatively long curing time when the composition is required to be cured at low temperatures. Especially, when the composition is applied onto plastic films such as polyethylene terephthalate and polypropylene films, the curing at low temperatures is essential for preventing shrinkage of the film during the curing. In addition, such low temperature curing over a long term may frequently result in rather poor adhesion to the film substrate. In an extreme case where the silicone composition is cured under high humidity conditions, the resultant layer of the composition is inconvenientlly liable to fall off in rags when rubbed with a finger about one day after the curing.
For the purpose of establishing good adhesion of the organopolysiloxane composition to plastic films, attempts have been made to add a certain type of silane or siloxane to the composition, thereby imparting self-adhesiveness or stickiness to the composition (such as disclosed in Japanese Laid-open Patent Application No. 46-2187 and Japanese Patent Publication No. 2-9464). In this type of composition, curing is completed based on a condensation reaction, which requires high temperatures for the curing. Such high temperatures are not favorable when plastic films are used as a substrate.
Japanese Laid-open Patent Application Nos. 52-24258 and 52-147657 and Japanese Patent Publication No. 57-1471 describe compositions in which a siloxane compound having epoxy groups at ends or side chains thereof is used and cured by its addition reaction. However, this compound also requires high curing temperatures, and the resultant cured film does not exhibit non-adhesiveness and is unstable. In addition, the adhesiveness to substrate is not sufficient and thus, this composition is not satisfactory for use with plastic films.
For improving the adhesiveness, a substrate or adherend may be treated with a primer. This treatment is additional and is not convenient in view of the working time, labor and cost.